Multi-piece solid golf ball

ABSTRACT

In a multi-piece solid golf ball having a rubber two-layer core consisting of an inner layer and an outer layer, a cover, and at least one intermediate layer therebetween, the inner core layer has a diameter of at least 22 mm, the difference in JIS-C hardness between the center of the inner core layer and the surface of the outer core layer is at least 25, the ball satisfies the relationship A/B≤2.0, where A and B are the deflections of, respectively, the inner core layer and the two-layer core when compressed under specific loading conditions, and the intermediate layer has a higher material hardness than the cover, and the core hardness profile are optimized by the specific conditions. This ball enables mid- to high-level golfers both to achieve even longer distances and to maintain the spin performance on approach shots at a high level.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 15/250,256 filed on Aug. 29, 2016, and claims benefit to JapanesePatent Application JP2015-172780, filed Sep. 2, 2015, the entirecontents of each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball which hasa rubber two-layer core consisting of an inner layer and an outer layer,a cover, and at least one intermediate layer between the core and thecover.

BACKGROUND ART

Recently, various golf balls have been proposed that attempt to achievethe intended spin properties and increase the distance of the ball, bothby providing the golf ball with a multilayer structure, and also byimparting the core that makes up most of the ball with a specifichardness profile in such a way as to optimize the core hardness profileand the overall hardness and thickness parameters of the ball. Art hasalso been proposed which, as a means for optimizing the core hardnessprofile, from the standpoint of the materials and manufacturing methodused, provides the core with a two-layer structure and satisfies adesired core hardness profile. Golf balls in which the core is made oftwo layers are described in, for example, U.S. Pat. Nos. 6,913,547,7,115,049, 7,267,621, 7,503,855, 7,175,542, 7,367,901, 7,625,302 and8,702,535.

However, among mid- to high-level amateur golfers and professionalshaving intermediate to high head speeds, there is a desire for thecommercialization of golf balls having a two-layer core that achieve alonger distance and that also, to further increase the enjoyability ofthe game, maintain the spin performance on approach shots at a highlevel.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a multi-piecesolid golf ball which, when used by mid- to high-level amateur golfersand professionals having intermediate to high head speeds, makes itpossible to achieve a lower spin rate and a high initial velocity onfull shots, enabling a good distance to be obtained when hit with adriver (W#1), and moreover which, to increase the enjoyability of thegame, also enables the spin performance on approach shots to bemaintained at a high level.

As a result of extensive investigations, we have discovered that, in theball construction of a multi-piece solid golf ball having a core, acover, and at least one intermediate layer therebetween, by using a corewhich is made of rubber and has a two-layer construction that is soft onthe inside and hard on the outside, this being accomplished by givingthe inner core layer a relatively large size with a diameter of at least22 mm, the spin rate on full shots can be made lower than in golf ballsin which the inner core layer has a small diameter. That is, by havingthe diameter of the inner core layer be at least 22 mm, by having thevalue obtained by subtracting the center hardness of the inner corelayer from the surface hardness of the outer core layer, expressed interms of JIS-C hardness, be at least 25, and by satisfying the numericalrelationship A/B≤2.0, where A is the deflection (mm) of the inner corelayer when compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf) and B is the deflection (mm) of thetwo-layer core when compressed under a final load of 1,275 N (130 kgf)from an initial load of 98 N (10 kgf), and moreover by forming anintermediate layer and a cover such that the material hardness of theintermediate layer is harder than the material hardness of the cover, itis possible, particularly when the ball is used by mid- to high-levelamateur golfers and professionals, both to suppress the spin rate onfull shots and thus further increase the distance and also to elevatethe spin performance on approach shots, enhancing the enjoyability ofplaying the ball in the short game.

As used herein, “mid- to high-level amateur golfer” refers to amateurgolfers having head speeds (HS) of generally 40 to 50 m/s, withmid-level amateur golfers having head speeds of about 40 to 48 m/s andhigh-level amateur golfers having head speeds of about 42 to 50 m/s.

Accordingly, the invention provides a multi-piece solid golf ball havinga rubber two-layer core consisting of an inner layer and an outer layer,a cover, and at least one intermediate layer between the core and thecover, wherein the inner core layer has a diameter of at least 22 mm,the value obtained by subtracting a center hardness of the inner corelayer from a surface hardness of the outer core layer, expressed interms of JIS-C hardness, is at least 25, the ball satisfies therelationship A/B≤2.0, where A is the deflection (mm) of the inner corelayer when compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf) and B is the deflection (mm) of thetwo-layer core when compressed under a final load of 1,275 N (130 kgf)from an initial load of 98 N (10 kgf), and the intermediate layer has ahigher material hardness than the cover, wherein the multi-piece solidgolf ball satisfies the conditions:

C5−Cc≤Cs−C15

C10−C5≤Cs−C15

C5−Cc≤C15−C10, and

1.5≤(Cs−C10)/(C10−Cc)≤4,

where Cc is the JIS-C hardness at a center of the core, C5 is the JIS-Chardness at a position 5 mm from the core center, C10 is the JIS-Chardness at a position 10 mm from the core center, C15 is the JIS-Chardness at a position 15 mm from the core center, and Cs is the JIS-Chardness at a surface of the core.

In a preferred embodiment of the inventive golf ball, the core has ahardness profile which satisfies the conditions:

1≤C10−Cc≤15,  (i)

C10−Cc<Cs−C10,  (ii)

18≤Cs−C10,  (iii)

Cs≥80, and  (iv)

Cc≥50.  (v)

In another preferred embodiment, the golf ball satisfies the condition:

25≤Cs−Cc≤45.  (vii)

In still another preferred embodiment, the golf ball satisfies thecondition:

C10−C5≤C5−Cc≤Cs−C15≤C15−C10.  (viii)

In a further preferred embodiment, the golf ball satisfies therelationship A/C≤1.9, where C is the deflection (mm) of a sphereconsisting of the core encased by the intermediate layer when the sphereis compressed under a final load of 1,275 N (130 kgf) from an initialload of 98 N (10 kgf).

In yet a further preferred embodiment, the golf ball satisfies therelationships A/H≤2.0 and A−H≤2.5, where H is the deflection (mm) of theball when compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf).

In a still further preferred embodiment, the golf ball satisfies thecondition:

PS ₇ /S/H×100≥5.90(mm⁻¹),  (ix)

where PS₇ is the pressed area (mm²), defined as the area of the golfball that comes into contact with a flat surface, when the ball issubjected to a load of 6,864 N (700 kgf), S is the hypothetical planararea (mm²), defined as the surface area of a cross-sectional circlealong the ball diameter were the surface of the ball to be entirely freeof dimples, and H is the deflection (mm) of the ball when compressedunder a final load of 1,275 N (130 kgf) from an initial load of 98 N (10kgf).

In yet another preferred embodiment, the golf ball satisfies thecondition:

PS ₂ /S/H×100≥1.70(mm⁻¹),  (x)

where PS₂ is the pressed area (mm²), defined as the area of the golfball that comes into contact with a flat surface, when the ball issubjected to a load of 1,961 N (200 kgf), S is the hypothetical planararea (mm²), defined as the surface area of a cross-sectional circlealong the ball diameter were the surface of the ball to be entirely freeof dimples, and H is the deflection (mm) of the ball when compressedunder a final load of 1,275 N (130 kgf) from an initial load of 98 N (10kgf).

Advantageous Effects of the Invention

The multi-piece solid golf ball of the invention, when used by mid- tohigh-level golfers, enables the spin rate on full shots with a driver(W#1) to be sufficiently lowered, making it possible to achieve anincreased distance, and also enables the spin performance on approachshots to be maintained at a high level.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIGS. 1A and 1B are enlarged cross-sectional diagrams of a dimple on thegolf balls used in Working Examples 1, 2, 4 and 5.

FIGS. 2A and 2B are enlarged cross-sectional diagrams of dimples on thegolf ball used in Working Example 3.

FIGS. 3A and 3B show explanatory diagrams for a method of determiningthe pressed area of a golf ball.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become moreapparent from the following detailed description, taken in conjunctionwith the foregoing diagram.

The multi-piece solid golf ball of the invention has a ball constructionwith a rubber two-layer core consisting of an inner layer and an outerlayer, a cover, and at least one intermediate layer between the core andthe cover.

The core, although not shown in the diagrams, is formed as two layers:an inner layer and an outer layer. The inner core layer has a diameterof at least 22 mm, preferably from 22 to 40 mm, more preferably from 30to 38 mm, and even more preferably from 35 to 35.5 mm. When the diameterof the inner core layer is too small, the initial velocity of the ballon shots with a driver (W#1) is low, as a result of which the intendeddistance may not be obtained. On the other hand, when the inner corelayer diameter is too large, the durability to cracking on repeatedimpact may worsen or the spin rate-lowering effect on full shots may beinadequate, as a result of which the intended distance may not beobtained.

The outer core layer has a thickness which, although not particularlylimited, is preferably from 1.0 to 10.0 mm, more preferably from 1.2 to8.0 mm, and even more preferably from 1.5 to 2.0 mm. When the outer corelayer is too thick, the initial velocity on full shots may be low, as aresult of which the intended distance may not be achieved. On the otherhand, when the outer core layer is too thin, the durability to crackingon repeated impact may worsen and the spin rate-lowering effect on fullshots may be inadequate, as a result of which the intended distance maynot be obtained.

The center hardness (CC) of the inner core layer described below (alsoreferred to as “the core center hardness”) and the cross-sectionalhardnesses at specific positions refer to hardnesses measured at thecenter and specific positions in a cross-section obtained by cutting thecore in half through the center. The surface hardness (Cs) refers to thehardness measured at the surface (spherical surface) of the core. Thesurface hardness (Cs) also refers to the surface hardness of the outercore layer.

The center hardness (Cc) of the inner core layer, expressed in terms ofJIS-C hardness, is preferably at least 50, more preferably from 51 to57, and even more preferably from 52 to 55. When the center hardness ofthe inner core layer is too large, the spin rate may rise excessively,as a result of which a good distance may not be achieved, and the ballmay have a hard feel at impact. On the other hand, when the centerhardness is too small, the durability to cracking on repeated impact mayworsen and the feel of the ball at impact may be too soft.

The JIS-C hardness at a position 5 mm from the center of the inner corelayer (C5) is preferably from 56 to 66, more preferably from 58 to 64,and even more preferably from 60 to 62. The JIS-C hardness at a position10 mm from the center of the inner core layer (C10) is preferably from59 to 72, more preferably from 61 to 67, and even more preferably from63 to 65. When these hardness values are too large, the spin rate mayrise excessively, as a result of which a good distance may not beobtained, or the feel at impact may be hard. On the other hand, whenthese values are too small, the durability to cracking on repeatedimpact may worsen and the feel at impact may become too soft.

The JIS-C hardness at a position 15 mm from the center of the inner corelayer (C15) is preferably from 73 to 83, more preferably from 75 to 81,and even more preferably from 75 to 80. When this hardness value is toohigh, the feel at impact may be hard and the durability to cracking onrepeated impact may worsen. On the other hand, when this hardness valueis too low, the spin rate may rise excessively and the rebound maydecrease, as a result of which a good distance may not be obtained.

The C10−Cc value is preferably from 1 to 15, more preferably from 5 to14, and even more preferably from 10 to 13. This value means that, fromthe core center out to about 10 mm, the hardness profile does not have avery steep gradient. The C5−Cc value is preferably from 4 to 11, morepreferably from 5 to 10, and even more preferably from 6 to 9. When thisvalue is too large, the initial velocity on full shots may be low andthe intended distance may not be obtained. On the other hand, when thisvalue is too small, the spin rate on full shots may become high and theintended distance may not be obtained.

The C10−C5 value is preferably from 1 to 7, more preferably from 2 to 6,and even more preferably from 3 to 5. When the C10−C5 value fallsoutside of this range, the spin rate on full shots may rise excessivelyand a good distance may not be obtained, or the durability to crackingon repeated impact may worsen.

The surface hardness of the outer core layer (Cs), expressed in terms ofJIS-C hardness, is preferably at least 80, more preferably from 81 to95, and even more preferably from 82 to 93. When the surface hardness ofthis outer core layer is too large, the feel at impact may harden, orthe durability to cracking on repeated impact may worsen. On the otherhand, when this value is too small, the spin rate may rise excessively,or the rebound may be low and a good distance may not be obtained.

The hardness difference between the surface hardness of the outer corelayer and the center hardness of the inner core layer (Cs−Cc), expressedin terms of JIS-C hardness, is preferably at least 25, more preferablyfrom 28 to 45, and even more preferably from 30 to 40. When thishardness difference is too large, the durability to cracking on repeatedimpact may worsen. On the other hand, when this hardness difference istoo small, the spin rate may rise excessively and a good distance maynot be achieved.

The Cs−C10 value is preferably at least 18, more preferably from 19 to35, and even more preferably from 21 to 30. This means that, from aposition 10 mm from the core center to the core surface, the hardnessprofile has a steep gradient that, in terms of JIS-C hardness, exceeds18. When this value is too large, the durability to cracking on repeatedimpact may worsen or the feel at impact may worsen. On the other hand,when this value is too small, the spin rate-lowering effect on fullshots may be inadequate, as a result of which the intended distance maynot be achieved.

The Cs−C10 value is preferably larger than the C10−Cc value. This meansthat, in the core hardness profile, the outside of the core has asteeper hardness gradient that the core interior. The value(Cs−C10)/(C10−Cc) is specified from 1.5 to 4.0, more preferably from 1.7to 3.3, and even more preferably from 2.0 to 2.6. When this value is toolarge, the durability to cracking on repeated impact may worsen. On theother hand, when this value is too small, the spin rate-lowering effecton full shots may be inadequate, as a result of which the intendeddistance may not be obtained.

In the above core hardness profile, it is necessary for the followingcondition to be satisfied:

C5−Cc≤Cs−C15

C10−C5≤Cs−C15 and

C5−Cc≤C15−C10.

When the above three relationships are not satisfied, the spinrate-lowering effects on full shots may be inadequate, or the initialvelocity on actual shots may be low, as a result of which the intendeddistance may not be obtained.

Furthermore, in the above core hardness profile, it is preferable forthe following condition to be satisfied:

C10−C5≤C5−Cc≤Cs−C15≤C15−C10

When this relationship is not satisfied, the spin rate-lowering effectson full shots may be inadequate, or the initial velocity on actual shotsmay be low, as a result of which the intended distance may not beobtained.

The inner core layer has a deflection (mm) when compressed under a finalload of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which,although not particularly limited, is preferably from 3.6 to 6.5 mm,more preferably from 3.9 to 4.8 mm, and even more preferably from 4.2 to4.5 mm. Also, the sphere obtained by encasing the inner core layer withthe outer core layer, i.e., the overall core, has a deflection (mm)under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10kgf) which, although not particularly limited, is preferably from 3.0 to4.2 mm, more preferably from 3.3 to 4.0 mm, and even more preferablyfrom 3.5 to 3.8 mm. When this value is too large, the feel at impact maybe too soft, the durability to repeated impact may worsen, or theinitial velocity on full shots may be low, as a result of which theintended distance may not be obtained. When this value is too small, thefeel at impact may be too hard or the spin rate on full shots mayincrease, as a result of which the intended distance may not beobtained.

In this invention, letting A be the deflection (mm) of the inner corelayer when compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf) and B be the deflection (mm) of thetwo-layer core when compressed under a final load of 1,275 N (130 kgf)from an initial load of 98 N (10 kgf), it is essential that the valueA/B be 2.0 or less, preferably from 0.8 to 1.8, and more preferably from1.0 to 1.3. When this value is too small, the feel at impact may be toosoft and the initial velocity on full shots may be low, as a result ofwhich the intended distance on shots with a driver (W#1) may not beobtained. On the other hand, when the A/B value is too large, the feelat impact may be too hard and the spin rate on full shots may riseexcessively, as a result of which the intended distance on shots with adriver (W#1) may not be obtained.

The inner core layer and outer core layer which have the above hardnessprofiles and deflections are preferably made of materials that arecomposed primarily of rubber. The rubber material making up the outercore layer encasing the inner core layer may be the same as or differentfrom the rubber material making up the inner core layer. For example,use may be made of a rubber composition obtained by compounding a baserubber as the chief component and, together with this, other ingredientssuch as a co-crosslinking agent, an organic peroxide, an inert fillerand an organosulfur compound.

Polybutadiene is preferably used as the base rubber. The polybutadienehas a cis-1,4 bond content on the polymer chain of typically at least 60wt %, preferably at least 80 wt %, more preferably at least 90 wt %, andmost preferably at least 95 wt %. When the content of cis-1,4 bondsamong the bonds on the polybutadiene molecule is too low, the resiliencemay decrease.

Rubber components other than this polybutadiene may be included in thebase rubber within a range that does not detract from the advantageouseffects of the invention. Examples of such rubber components other thanthe foregoing polybutadiene include other polybutadienes, and dienerubbers other than polybutadiene, such as styrene-butadiene rubber,natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

The organic peroxide used in the invention is not particularly limited,although the use of an organic peroxide having a one-minute half-lifetemperature of 110 to 185° C. is preferred. One, two or more organicperoxides may be used. The amount of organic peroxide included per 100parts by weight of the base rubber is preferably at least 0.1 part byweight, and more preferably at least 0.3 part by weight. The upper limitis preferably not more than 5 parts by weight, more preferably not morethan 4 parts by weight, and even more preferably not more than 3 partsby weight. A commercially available product may be used as the organicperoxide. Specific examples include those available under the tradenames Percumyl D, Perhexa C-40, Niper B W and Peroyl L (all from NOFCorporation), and Luperco 231XL (from Atochem Co.).

The co-crosslinking agent is exemplified by unsaturated carboxylic acidsand the metal salts of unsaturated carboxylic acids. Illustrativeexamples of unsaturated carboxylic acids include acrylic acid,methacrylic acid, maleic acid and fumaric acid. Acrylic acid andmethacrylic acid are especially preferred. Metal salts of unsaturatedcarboxylic acids are not particularly limited, and are exemplified bythose obtained by neutralizing the foregoing unsaturated carboxylicacids with the desired metal ions. Illustrative examples include thezinc salts and magnesium salts of methacrylic acid and acrylic acid. Theuse of zinc acrylate is especially preferred.

These unsaturated carboxylic acids and/or metal salts thereof areincluded in an amount per 100 parts by weight of the base rubber whichis typically at least 10 parts by weight, preferably at least 15 partsby weight, and more preferably at least 20 parts by weight. The upperlimit is typically not more than 60 parts by weight, preferably not morethan 50 parts by weight, more preferably not more than 45 parts byweight, and most preferably not more than 40 parts by weight. When toomuch is included, the feel of the ball may become too hard andunpleasant. When too little is included, the rebound may decrease.

In order to have the core satisfy the desired hardness profile describedabove, water or a water-containing material may be added whencompounding the various ingredients of the core-forming rubbercomposition. Decomposition of the organic peroxide within the coreformulation can be promoted by the direct addition of water (or awater-containing material) to the core material. It is known that thedecomposition efficiency of the organic peroxide within the core-formingrubber composition changes with temperature and that, starting at agiven temperature, the decomposition efficiency rises with increasingtemperature. If the temperature is too high, the amount of decomposedradicals rises excessively, leading to recombination between radicalsand, ultimately, deactivation. As a result, fewer radicals acteffectively in crosslinking. Here, when a heat of decomposition isgenerated by decomposition of the organic peroxide at the time of corevulcanization, the vicinity of the core surface remains at substantiallythe same temperature as the temperature of the vulcanization mold, butthe temperature near the core center, due to the build-up of heat ofdecomposition by the organic peroxide which has decomposed from theoutside, becomes considerably higher than the mold temperature. In caseswhere water (or a water-containing material) is added directly to thecore, because the water acts to promote decomposition of the organicperoxide, radical reactions like those described above can be made todiffer at the core center and at the core surface. That is,decomposition of the organic peroxide is further promoted near thecenter of the core, bringing about greater radical deactivation, whichleads to a further decrease in the amount of active radicals. As aresult, it is possible to obtain a core in which the crosslink densitiesat the core center and the core surface differ markedly. It is alsopossible to obtain a core having different dynamic viscoelasticproperties at the core center. Along with achieving a lower spin rate,golf balls having such a core are also able to exhibit excellentdurability and undergo less change over time in rebound. When zincmonoacrylate is used instead of the above water, water is generated fromthe zinc monoacrylate by heat during kneading of the compoundingmaterials. An effect similar to that obtained by the addition of watercan thereby be obtained.

The water used here is not particularly limited, and may be distilledwater or tap water. The use of distilled water which is free ofimpurities is especially preferred. The amount of water included per 100parts by weight of the base rubber is preferably at least 0.1 part byweight, and more preferably at least 0.3 part by weight. The upper limitis preferably not more than 5 parts by weight, and more preferably notmore than 4 parts by weight.

The production of such a core composed of two layers may entail moldingan inner core layer by, for example, the customary method of forming asphere under heating and compression at a temperature of at least 140°C. but not more than 180° C. for a period of at least 10 minutes but notmore than 60 minutes. The method employed to form the outer core layeron the surface of the inner core layer may involve forming a pair ofhalf-cups from unvulcanized rubber sheet, placing and enclosing theinner core layer within the pair of half-cups, then molding under heatand pressure. For example, advantageous use may be made of a process inwhich initial vulcanization (semi-vulcanization) is carried out toproduce a pair of hemispherical cups, following which a prefabricatedinner core layer is placed in one of the hemispherical cups and coveredby the other hemispherical cup, and secondary vulcanization (completevulcanization) is subsequently carried out. Another preferred productionprocess involves forming the rubber composition while in an unvulcanizedstate into sheets so as to make a pair of outer core layer sheets, andshaping the sheets with a die having a hemispherical protrusion so as toproduce unvulcanized hemispherical cups. The pair of hemispherical cupsis then placed over a prefabricated inner core layer and formed into aspherical shape under heating and compression at a temperature of 140 to180° C. for a period of 10 to 60 minutes, thereby dividing thevulcanization step into two stages.

Next, the intermediate layer is described.

The intermediate layer has a material hardness expressed in terms ofShore D hardness which, although not particularly limited, is preferablyfrom 57 to 70, more preferably from 59 to 65, and even more preferablyfrom 61 to 63. The sphere consisting of the core encased by theintermediate layer, referred to below as the “intermediate layer-encasedsphere,” has a surface hardness, expressed in terms of Shore D hardness,which is preferably from 64 to 77, more preferably from 66 to 72, andeven more preferably from 68 to 70. When the intermediate layer is toosoft, the spin rate on full shots may rise excessively, as a result ofwhich a good distance may not be achieved. On the other hand, when theintermediate layer is too hard, the durability to cracking on repeatedimpact may worsen and the feel of the ball on shots with a putter or onshort approaches may become too hard.

The intermediate layer-encased sphere has a deflection (mm) whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf) which, although not particularly limited, is preferablyfrom 2.4 to 3.6 mm, more preferably from 2.6 to 3.4 mm, and even morepreferably from 2.8 to 3.1 mm. When this value is too large, the feel ofthe ball may be too soft, the durability to repeated impact may be poor,and the initial velocity on full shots may be low, as a result of whichthe intended distance may not be achieved. On the other hand, when thisvalue is too small, the feel of the ball may be too hard and the spinrate on full shots may rise, as a result of which the intended distancemay not be achieved.

The value obtained by subtracting the surface hardness of the outer corelayer from the surface hardness of the intermediate layer-encasedsphere, expressed in terms of JIS-C hardness, is preferably from 1 to20, more preferably from 3 to 16, and even more preferably from 5 to 13.When this value falls outside of the foregoing range, the spinrate-lowering effect on full shots may be inadequate, as a result ofwhich the intended distance may not be obtained, or the durability tocracking on repeated impact may worsen.

Letting C be the deflection (mm) of the intermediate layer-encasedsphere when compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf), the value A/C is preferably not more than1.9, more preferably from 1.2 to 1.7, and even more preferably from 1.4to 1.5. When this value is too small, the feel at impact may be too softand the initial velocity on full shots may be too low, as a result ofwhich the intended distance on shots with a driver (W#1) may not beobtained. On the other hand, when the value A/C is too large, the feelat impact may be too hard and the spin rate on full shots may riseexcessively, as a result of which the intended distance on shots with adriver (W#1) may not be obtained.

The intermediate layer has a thickness of preferably from 0.8 to 2.1 mm,more preferably from 1.0 to 1.7 mm, and even more preferably from 1.2 to1.4 mm. The thickness of the intermediate layer is preferably higherthan the thickness of the subsequently described cover (outermostlayer). When the intermediate layer thickness falls outside of thisrange or is thinner than the cover, the spin rate-lowering effect onshots with a driver (W#1) may be inadequate, as a result of which a gooddistance may not be achieved.

The intermediate layer material is not particularly limited, althoughpreferred use can be made of various thermoplastic resin materials. Tofully achieve the desired effects of the invention, it is especiallypreferable to use a high-resilience resin material as the intermediatelayer material. For example, the use of an ionomer resin material or thesubsequently described highly neutralized resin material is preferred.

Specifically, a molded material obtained by molding a resin compositionof components (I) to (IV) described below under applied heat may be usedas the highly neutralized resin material.

Preferred use can be made of the two following components (I) and (II)as the base resins:

-   (I) An olefin-unsaturated carboxylic acid-unsaturated carboxylic    acid ester terpolymer, or a metal neutralization product thereof,    having a weight-average molecular weight (Mw) of at least 140,000,    an acid content of 10 to 15 wt % and an ester content of at least 15    wt %; and-   (II) An olefin-acrylic acid random copolymer, or a metal    neutralization product thereof, having a weight-average molecular    weight (Mw) of at least 140,000 and an acid content of 10 to 15 wt    %.

The weight-average molecular weight (Mw) of component (I) is at least140,000, and preferably at least 145,000. The weight-average molecularweight (Mw) of component (II) is at least 140,000, and preferably atleast 160,000. By thus making these molecular weights large, the resinmaterial can be assured of having sufficient resilience.

It is thought that because the acid components and ester contents of therespective copolymers serving as the base resins (I) and (II) differ,these two types of base resins interlock in a complex manner, givingrise to molecular synergistic effects that can increase the rebound anddurability of the ball. In this invention, by specifying theweight-average molecular weight, acid content and ester content asindicated above in such a way as to select a material that is relativelysoft as the terpolymer serving as base resin (I), and by specifying thetype of acid, weight-average molecular weight and acid content in such away as to select a relatively hard material as base resin (II), it ispossible with a blend of these polymers to ensure sufficient resilienceand durability for use as a golf ball material.

Here, the weight-average molecular weight (Mw) is a value calculatedrelative to polystyrene in gel permeation chromatography (GPC). A wordof explanation is needed here concerning GPC molecular weightmeasurement. It is not possible to directly take GPC measurements forcopolymers and terpolymers because these molecules are adsorbed to theGPC column owing to unsaturated carboxylic acid groups within themolecules. Instead, the unsaturated carboxylic acid groups are generallyconverted to esters, following which GPC measurement is carried out andthe polystyrene-equivalent average molecular weights Mw and Mn arecalculated.

The olefins used in component (I) and component (II) preferably have 2to 6 carbons, with ethylene being especially preferred. The unsaturatedcarboxylic acid used in component (I) is not particularly limited,although preferred use can be made of acrylic acid or methacrylic acid.To ensure resilience, the unsaturated carboxylic acid used in component(II) is acrylic acid. This is because, when methacrylic acid is used asthe unsaturated carboxylic acid in component (II), the methacrylic acidwith its pendant methyl group may give rise to a buffering action,lowering the reactivity.

The unsaturated carboxylic acid content (acid content) within each ofcomponents (I) and (II), although not particularly limited, ispreferably at least 10 wt %, with the upper limit being preferably lessthan 15 wt %, and more preferably less than 13 wt %. When this acidcontent is low, moldings of the golf ball material may lack sufficientresilience. On the other hand, when the acid content is high, thehardness may become excessively high, adversely affecting thedurability.

The unsaturated carboxylic acid ester used in the terpolymer serving ascomponent (I) is preferably a lower alkyl ester, with butyl acrylate(butyl n-acrylate, butyl i-acrylate) being especially preferred.

The ester content of the unsaturated carboxylic acid ester in component(I), in order to employ a resin that is relatively soft compared withthe binary copolymer serving as component (II), is set to at least 15 wt%, preferably at least 18 wt %, and more preferably at least 20 wt %,with the upper limit being preferably not more than 25 wt %. At an estercontent higher than this range, moldings of the intermediate layermaterial may lack sufficient resilience. On the other hand, when theester content is low, the hardness may increase, adversely affecting thedurability.

The hardness of the base resin (I), that is, the hardness when the resinitself is molded alone (material hardness), expressed in terms of ShoreD hardness, is preferably at least 30, and more preferably at least 35,with the upper limit being preferably not more than 50, and morepreferably not more than 45. The hardness of the base resin (II), thatis, the hardness when the resin itself is molded alone (materialhardness), expressed in terms of Shore D hardness, is preferably atleast 40, and more preferably at least 50, with the upper limit beingpreferably not more than 60, and more preferably not more than 57. Whenbase resins outside of these respective hardness ranges are used, amaterial having the desired hardness may not be obtained, or an adequateresilience and durability may not be obtained.

It is preferable for component (I) and component (II) to be usedtogether. The mixing proportions of component (I) and component (II),expressed as the weight ratio (I):(II), is set to preferably 90:10 to10:90, more preferably 85:15 to 30:70, and even more preferably 80:20 to50:50. When the proportion of component II is higher than this range,the hardness increases, as a result of which the material may bedifficult to mold.

When metal neutralization products of resins (i.e., ionomers) are usedas components (I) and (II), the type of metal neutralization product andthe degree of neutralization are not particularly limited. Illustrativeexamples include 60 mol % Zn (degree of neutralization with zinc)ethylene-methacrylic acid copolymers, 40 mol % Mg (degree ofneutralization with magnesium) ethylene-methacrylic acid copolymers, and40 mol % Mg (degree of neutralization with magnesium)ethylene-methacrylic acid-acrylic acid ester terpolymers.

To ensure at least a given degree of flowability during injectionmolding and provide a good molding processability, it is essential forthe melt flow rates of the resins serving as components (I) and (II) toeach be from 0.5 to 20 g/10 min. The difference between the melt flowrates of components (I) and (II) is set to not more than 15 g/10 min.When the difference in melt flow rates between these base resins is toolarge, the components cannot be uniformly mixed together during thecompounding of components (I) and (II) in an extruder, and so themixture becomes non-uniform, which may lead to injection moldingdefects.

As noted above, copolymers or ionomers with weight-average molecularweights (Mw) set in specific ranges are used as components (I) and (II).Illustrative examples of commercial products that may be used for thispurpose include the Nucrel series (DuPont-Mitsui Polychemicals Co.,Ltd.), the Escor series (ExxonMobil Chemical), the Surlyn series (E.I.DuPont de Nemours & Co.), and the Himilan series (DuPont-MitsuiPolychemicals Co., Ltd.).

In addition, (III) a basic inorganic metal compound is preferablyincluded as a component for neutralizing acid groups in above components(I) and (II) and subsequently described component (IV). By even morehighly neutralizing the resin material in this way, the spin rate of theball on full shots is even further reduced without adversely affectingthe feel of the ball, thus making an increased distance fullyachievable. Illustrative examples of the metal ions in the basicinorganic metal compound include Na⁺, K⁺, Li⁺, Zn²⁺, Ca²⁺, Mg²⁺, Cu²⁺and Co²⁺. Of these, Na⁺, Zn²⁺, Ca²⁺ and Mg²⁺ are preferred, and Mg²⁺ ismore preferred. These metal salts may be introduced into the resinusing, for example, formates, acetates, nitrates, carbonates,bicarbonates, oxides and hydroxides.

This basic inorganic metal compound (III) is included in the resincomposition in an amount equivalent to at least 70 mol %, based on theacid groups in the resin composition. Here, the amount in which thebasic inorganic metal compound serving as component (III) is includedmay be selected as appropriate for obtaining the desired degree ofneutralization. Although this amount depends also on the degree ofneutralization of the base resins (components (I) and (II)) that areused, in general it is preferably from 1.0 to 2.5 parts by weight, morepreferably from 1.1 to 2.3 parts by weight, and even more preferablyfrom 1.2 to 2.0 parts by weight, per 100 parts by weight of the combinedamount of the base resins (components (I) and (II)). The degree ofneutralization of the acid groups in components (I) to (IV) ispreferably at least 70 mol %, more preferably at least 90 mol %, andeven more preferably at least 100 mol %.

Next, the anionic surfactant serving as component (IV) is described. Thereason for including an anionic surfactant is to improve the durabilityafter resin molding while ensuring good flowability of the overall resincomposition. The anionic surfactant is not particularly limited,although the use of one having a molecular weight of from 140 to 1,500is preferred. Exemplary anionic surfactants include carboxylatesurfactants, sulfonate surfactants, sulfate ester surfactants andphosphate ester surfactants. Preferred examples include one, two or moreselected from the group consisting of various fatty acids such asstearic acid, behenic acid, oleic acid and maleic acid, derivatives ofthese fatty acids, and metal salts thereof. Selection from the groupconsisting of stearic acid, oleic acid and mixtures thereof isespecially preferred. Alternatively, exemplary organic acid metal saltsthat may serve as component (IV) include metal soaps, with the metalsalt being one in which a metal ion having a valence of 1 to 3 is used.The metal is preferably selected from the group consisting of lithium,sodium, magnesium, aluminum, potassium, calcium and zinc, with the useof metal salts of stearic acid being especially preferred. Specifically,the use of magnesium stearate, calcium stearate, zinc stearate or sodiumstearate is preferred.

Component (IV) is included in an amount, per 100 parts by weight of thebase resins serving as components (I) and (II), of 1 to 100 parts byweight, preferably 10 to 90 parts by weight, and more preferably 20 to80 parts by weight. When the component (IV) content is too low, it maybe difficult to lower the hardness of the resin material. On the otherhand, at a high content, the resin material is difficult to mold andbleeding at the material surface increases, adversely affecting themolded article.

In this invention, the moldability of the material and the productivitycan be further increased by suitably adjusting the compounding ratiobetween components (III) and (IV). When the content of the basicinorganic metal compound serving as component (III) is too high, theamount of gases such as organic acids that evolve during moldingdecreases, but the flowability of the material diminishes. Conversely,when the content of component (III) is low, the amount of gasesgenerated increases. On the other hand, when the content of the anionicsurfactant serving as component (IV) is too high, the amount of gasconsisting of fatty acids and other organic acids increases duringmolding, which has a large impact in terms of molding defects andproductivity. Conversely, when the content of component (IV) is low, theamount of gases generated decreases, but the flowability and durabilitydecline. Therefore, achieving a proper compounding balance betweencomponents (III) and (IV) is also important. Specifically, it isdesirable to set the compounding ratio between components (III) and(IV), expressed as the weight ratio (III):(IV), to from 4.0:96.0 to1.0:99.0, and especially from 3.0:97.0 to 1.5:98.5.

The resin composition of above components (I) to (IV) accounts forpreferably at least 50 wt %, more preferably at least 60 wt %, even morepreferably at least 70 wt %, and most preferably at least 90 wt %, ofthe total amount of the intermediate layer material.

A non-ionomeric thermoplastic elastomer may be included in theintermediate layer material. The non-ionomeric thermoplastic elastomeris preferably included in an amount of from 1 to 50 parts by weight per100 parts by weight of the combined amount of the base resins.

The non-ionomeric thermoplastic elastomer is exemplified by polyolefinelastomers (including polyolefins and metallocene-catalyzedpolyolefins), polystyrene elastomers, diene polymers, polyacrylatepolymers, polyamide elastomers, polyurethane elastomers, polyesterelastomers and polyacetals.

Optional additives may be suitably included in the intermediate layermaterial according to the intended use. For example, various additivessuch as pigments, dispersants, antioxidants, ultraviolet absorbers andlight stabilizers may be added. When such additives are included, thecontent thereof per 100 parts by weight of components (I) to (IV)combined is preferably at least 0.1 part by weight, and more preferablyat least 0.5 part by weight, with the upper limit being preferably notmore than 10 parts by weight, and more preferably not more than 4 partsby weight.

It is desirable to abrade the surface of the intermediate layer in orderto increase adhesion with the polyurethane that is preferably used inthe subsequently described cover (outermost layer). In addition, it isdesirable to apply a primer (adhesive) to the surface of theintermediate layer following such abrasion treatment or to add anadhesion reinforcing agent to the intermediate layer material.

The intermediate layer material has a specific gravity which istypically less than 1.1, preferably from 0.90 to 1.05, and morepreferably from 0.93 to 0.99. Outside of this range, the rebound becomessmall, as a result of which a good distance may not be obtained, or thedurability to cracking on repeated impact may worsen.

Next, the cover, which is the outermost layer of the ball, is described.

The cover (outermost layer) has a material hardness expressed in termsof Shore D hardness which, although not particularly limited, ispreferably from 34 to 58, more preferably from 40 to 56, and even morepreferably from 48 to 54.

The cover-encased sphere, i.e., the ball, has a surface hardnessexpressed in terms of Shore D hardness which is preferably from 40 to70, more preferably from 46 to 68, and even more preferably from 54 to66. When the cover-encased sphere is softer than this range, the spinrate on driver (W#1) shots and iron shots may become too high, as aresult of which a good distance may not be obtained. When the cover isharder than this range, the spin rate on approach shots may beinadequate or the feel at impact may be too hard.

The value obtained by subtracting the surface hardness of theintermediate layer-encased sphere from the surface hardness of the ball,expressed in terms of Shore D hardness, is preferably from −12 to −1,more preferably from −9 to −2, and even more preferably from −5 to −3.When this value is larger (less negative), the ball may be lessreceptive to spin on approach shots, or the durability to cracking onrepeated impact may worsen. On the other hand, when this value is toosmall (larger in the negative direction), the spin rate on full shotsmay increase, or the initial velocity of the ball may decrease, as aresult of which the intended distance may not be achieved.

The cover-encased sphere, i.e., the ball, has a deflection (mm) whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf) which, although not particularly limited, is preferablyfrom 2.2 to 3.5 mm, more preferably from 2.4 to 3.2 mm, and even morepreferably from 2.6 to 2.9 mm. When this value is too large, the feel ofthe ball may be too soft, the durability to repeated impact may worsen,or the initial velocity on full shots may be low, as a result of whichthe intended distance may not be obtained. On the other hand, when thisvalue is too small, the feel of the ball may be too hard and the spinrate on full shots may rise, as a result of which the intended distancemay not be obtained.

Letting H be the deflection (mm) of the golf ball when compressed undera final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf),the value A/H is preferably not more than 2.5, more preferably from 1.2to 1.8, and even more preferably from 1.5 to 1.6. When this value is toosmall, the feel at impact may become too soft and the initial velocityon full shots may be too low, as a result of which the intended distanceon shots with a driver (W#1) may not be obtained. On the other hand,when the value A/H is too large, the feel at impact may become too hardand the spin rate on full shots may rise excessively, as a result ofwhich the intended distance on shots with a driver may not be obtained.

The value A−H is preferably from 0.8 to 3.5 mm, even more preferablyfrom 1.0 to 2.4 mm, and still more preferably from 1.3 to 1.8 mm. Whenthis value is too small, the spin rate on full shots may riseexcessively and the intended distance on shots with a driver may not beobtained. On the other hand, when this value is too large, the initialvelocity on full shots may be too low, as a result of which the intendeddistance on driver shots may not be obtained.

The cover serving as the outermost layer of the ball has a thicknesswhich, although not particularly limited, is preferably from 0.3 to 1.5mm, more preferably from 0.45 to 1.2 mm, and even more preferably from0.6 to 0.9 mm. When the cover is thicker than this range, the rebound onW#1 shots and iron shots may be inadequate and the spin rate may rise,as a result of which a good distance may not be obtained. On the otherhand, when the cover is thinner than this range, the scuff resistancemay worsen and the ball may lack spin receptivity on approach shots,resulting in poor controllability.

It is preferable for the cover thickness to be smaller than thethickness of the intermediate layer; that is, for the intermediate layerto be formed so as to be thicker than the cover. The value obtained bysubtracting the cover thickness from the intermediate layer thickness ispreferably from 0.1 to 1.0 mm, more preferably from 0.2 to 0.8 mm, andeven more preferably from 0.3 to 0.6 mm. When this value is too large,the feel at impact may be too hard or the ball may have a poorreceptivity to spin on approach shots. On the other hand, when thisvalue is too small, the durability to cracking on repeated impact mayworsen, or the spin rate-lowering effect on full shots may beinadequate, as a result of which the intended distance may not beachieved.

The cover (outermost layer) material is not particularly limited,although any of various types of thermoplastic resin materials orthermoset resin materials may be used. For reasons having to do withcontrollability and scuff resistance, it is preferable to use a urethaneresin as the cover material. In particular, from the standpoint of themass productivity of manufactured golf balls, it is preferable to use acover material composed primarily of a thermoplastic polyurethane, withformation more preferably being carried out using a resin blend composedprimarily of (A) a thermoplastic polyurethane and (B) a polyisocyanatecompound.

In the thermoplastic polyurethane composition containing abovecomponents (A) and (B), to improve the ball properties even further, anecessary and sufficient amount of unreacted isocyanate groups should bepresent in the cover resin material. Specifically, it is recommendedthat the combined weight of components (A) and (B) be at least 60%, andmore preferably at least 70%, of the weight of the overall cover layer.

The thermoplastic polyurethane (A) has a structure which includes softsegments composed of a polymeric polyol (polymeric glycol) that is along-chain polyol, and hard segments composed of a chain extender and apolyisocyanate compound. Here, the long-chain polyol serving as astarting material may be any that has hitherto been used in the artrelating to thermoplastic polyurethanes, and is not particularlylimited. Illustrative examples include polyester polyols, polyetherpolyols, polycarbonate polyols, polyester polycarbonate polyols,polyolefin polyols, conjugated diene polymer-based polyols, castoroil-based polyols, silicone-based polyols and vinyl polymer-basedpolyols. These long-chain polyols may be used singly, or two or more maybe used in combination. Of these, in terms of being able to synthesize athermoplastic polyurethane having a high rebound resilience andexcellent low-temperature properties, a polyether polyol is preferred.

Any chain extender that has hitherto been employed in the art relatingto thermoplastic polyurethanes may be advantageously used as the chainextender. For example, low-molecular-weight compounds with a molecularweight of 400 or less which have on the molecule two or more activehydrogen atoms capable of reacting with isocyanate groups are preferred.Illustrative, non-limiting, examples of the chain extender include1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanedioland 2,2-dimethyl-1,3-propanediol. Of these, an aliphatic diol having 2to 12 carbons is preferred, and 1,4-butylene glycol is more preferred,as the chain extender.

Any polyisocyanate compound hitherto employed in the art relating tothermoplastic polyurethanes may be advantageously used withoutparticular limitation as the polyisocyanate compound. For example, usemay be made of one, two or more selected from the group consisting of4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, p-phenylene diisocyanate, xylylene diisocyanate,1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate,hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate,tetramethylene diisocyanate, hexamethylene diisocyanate, isophoronediisocyanate, norbornene diisocyanate, trimethylhexamethylenediisocyanate and dimer acid diisocyanate. However, depending on the typeof isocyanate, the crosslinking reaction during injection molding may bedifficult to control. To provide a balance between stability at the timeof production and the properties that are manifested, it is mostpreferable to use the following aromatic diisocyanate:4,4′-diphenylmethane diisocyanate.

Commercially available products may be used as the thermoplasticpolyurethane serving as component (A). Illustrative examples includePandex T-8295, T-8290, T-8283 and T-8260 (all from DIC Bayer Polymer,Ltd.).

Although not an essential ingredient, a thermoplastic elastomer (C)other than the above thermoplastic polyurethane may be included as anadditional component together with above components (A) and (B). Byincluding this component (C) in the above resin blend, a furtherimprovement in the flowability of the resin blend can be achieved andthe properties required of a golf ball cover material, such asresilience and scuff resistance, can be enhanced.

The relative proportions of above components (A), (B) and (C) are notparticularly limited. However, to fully elicit desirable effects withregard to resilience and the like, the weight ratio (A):(B):(C) ispreferably from 100:2:50 to 100:50:0, and more preferably from 100:2:50to 100:30:8.

In addition, various additives may be optionally included in the aboveresin blend. For example, pigments, dispersants, antioxidants, lightstabilizers, ultraviolet absorbers and internal mold lubricants may besuitably included.

The manufacture of multi-piece solid golf balls in which theabove-described two-layer core, intermediate layer and cover (outermostlayer) are formed as successive layers may be carried out by a customarymethod such as a known injection-molding process. For example, amulti-piece golf ball may be obtained by placing a two-layer core in agiven injection mold, injecting an intermediate layer material over thecore to give an intermediate sphere, and subsequently placing theresulting sphere in another injection mold and injection-molding a cover(outermost layer) material over the sphere. Alternatively, a cover(outermost layer) may be formed over the intermediate sphere by a methodthat involves encasing the intermediate sphere with a cover, this beingcarried out by, for example, enclosing the intermediate sphere withintwo half-cups that have been pre-molded into hemispherical shapes, andthen molding under applied heat and pressure.

It is preferable for the golf ball of the invention to satisfy thecondition:

PS ₇ /S/H×100≥5.90(mm⁻¹),

where PS₇ is the pressed area (mm²), defined as the area of the golfball that comes into contact with a flat surface, when the ball issubjected to a load of 6,864 N (700 kgf), S is the hypothetical planararea (mm²), defined as the surface area of a cross-sectional circlealong the ball diameter were the surface of the ball to be entirely freeof dimples, and H is the deflection (mm) of the ball when compressedunder a final load of 1,275 N (130 kgf) from an initial load of 98 N (10kgf).

By having the pressed area of the golf ball under loading on a drivershot by an ordinary golfer satisfy the above condition, the surface areaof contact between the ball and golf club increases and frictionalforces with the club rise, as a result of which the amount of back spinon driver shots decreases, enabling the distance to be improved.

It is preferable also for the golf ball of the invention to satisfy thecondition:

PS ₂ /S/H×100≥1.70(mm⁻¹),

where PS₂ is the pressed area (mm²), defined as the area of the golfball that comes into contact with a flat surface, when the ball issubjected to a load of 1,961 N (200 kgf), S is the hypothetical planararea (mm²), defined as the surface area of a cross-sectional circlealong the ball diameter were the surface of the ball to be entirely freeof dimples, and H is the deflection (mm) of the ball when compressedunder a final load of 1,275 N (130 kgf) from an initial load of 98 N (10kgf).

By having the pressed area of the golf ball under loading on an approachshot by an ordinary golfer satisfy the above condition, the surface areaof contact between the ball and golf club increases and frictionalforces with the club rise, as a result of which the amount of back spinon approach shots increases, enabling movement of the ball to be stoppedin a straighter line near the landing point of the ball.

The hypothetical planar surface area S of the golf ball is determined bythe ball diameter. The diameter may be set in conformity with the Rulesof Golf for play, this being of a size such that the ball does not passthrough a ring having an inner diameter of 42.672 mm and is not morethan 42.80 mm.

The pressed areas PS₇ and PS₂ of the golf ball under predetermined loadsrepresent the areas of contact by the golf ball with the golf club atthe time of a given shot. These areas of contact have been made largerthan in the prior art by means of the dimple structure. However, thepressed area PS is dependent on the size of the golf ball, becominglarger when the golf ball dimensions are larger and becoming smallerwhen the golf ball dimensions are smaller. Hence, by dividing thepressed area by the hypothetical planar surface area S and expressingthe result as a percentage, it is possible to evaluate the increase inthe area of contact due to the dimple construction without beingaffected by the size of the golf ball. The pressed area PS is alsodependent on the deflection H of the golf ball, becoming larger when thedeflection H is larger, and becoming smaller when the deflection H issmaller. Therefore, by dividing the pressed area by the deflection H, itis possible to evaluate the increase in the area of contact due to thedimple construction without being affected by the amount of deflectionby the golf ball. Measurement of the pressed area may be carried out by,for example, placing pressure-sensitive paper on a flat surface, settingthe golf ball to be tested on the paper, applying a load of 6,864 N (700kgf) or 1,961 (200 kgf) to the golf ball, and measuring the total areaof the portion of the pressure-sensitive paper that has become coloredas a result of contact with the golf ball. FIG. 3A shows an example ofpressure-sensitive paper that was actually colored when a load of 6,864N (700 kgf) was applied to a golf ball, and FIG. 3B shows an example ofpressure-sensitive paper that was actually colored when a load of 1,861N (200 kgf) was applied to the same golf ball. In these diagrams, theround areas are dimples, and the solid (blackened) places indicate thecolored portions. The area of the colored portions can be easilydetermined by using a commercial pressure image analysis system.

Numerous dimples may be formed on the cover (outermost layer). Thenumber of dimples arranged on the cover surface, although notparticularly limited, may be set to preferably at least 250, and morepreferably at least 300, with the upper limit being preferably not morethan 500, and more preferably not more than 450.

The dimple surface coverage SR (i.e., the ratio of the sum of theindividual dimple areas with respect to the total surface area of thehypothetical sphere were the ball assumed to have no dimples thereon) isset to preferably at least 70%, more preferably at least 75%, and evenmore preferably at least 80%. The maximum dimple surface coverage ratioSR, although not particularly limited, is preferably not more than 99%.It is especially desirable for the ball to be provided with at leastthree types of dimples of differing size, and for the dimples to bethereby uniformly arranged on the spherical surface of the ball withoutleaving gaps.

The dimple volume occupancy VR (i.e., the sum of the volumes of theindividual dimples, each formed below the flat plane circumscribed bythe edge of a dimple, expressed as a ratio with respect to the volume ofthe hypothetical sphere were the ball assumed to have no dimplesthereon) is set to preferably at least 0.75%, more preferably at least0.80%, and even more preferably at least 1.1%. The upper limit in thedimple volume occupancy VR is preferably not more than 1.5%, and morepreferably not more than 1.4%.

EXAMPLES

The following Examples and Comparative Examples are provided toillustrate the invention, and are not intended to limit the scopethereof.

Examples 1 to 5 Comparative Examples 1 to 3 Formation of Core

Inner core layers having the rubber compositions shown in Table 1 wereformed under the vulcanization temperatures and times shown in the sametable. Next, outer layer cores having the rubber compositions shown inTable 2 were formed over the inner core layers under the vulcanizationtemperatures and times shown in Table 2, thereby producing solid coresof rubber having the inner and outer core layers in the respectiveWorking Examples of the invention and the Comparative Examples.

TABLE 1 Comparative Example Example 1 2 3 4 5 1 2 3 Inner corePolybutadiene A 80 80 80 20 20 80 80 layer Polybutadiene B 20 20 20 8080 100 20 20 formulation Zinc acrylate 30 30 30 20.4 17.5 22 30 30 (pbw)Organic peroxide (1) 1 1 1 0.3 0.3 1 1 Organic peroxide (2) 0.3 0.3 1.2Water 0.8 0.8 0.8 0.8 0.8 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Barium sulfate 17.7 17.7 17.7 20.5 21.8 17.7 17.7 Zinc oxide 4 4 4 4 424.3 4 4 Zinc stearate 5 Zinc salt of 0.2 0.2 0.2 2 0.2 0.2pentachlorothiophenol Vulcanization Temperature (° C.) 155 155 155 155155 155 155 155 conditions Time (min.) 15 15 15 15 15 15 15 15

TABLE 2 Comparative Example Example 1 2 3 4 5 1 2 3 Outer corePolybutadiene A 100 100 100 20 20 100 100 100 layer Polybutadiene B 8080 formulation Zinc acrylate 32 29 29 35.6 32.5 33 21.5 32 (pbw) Organicperoxide (1) 0.6 0.6 0.6 0.6 0.6 Organic peroxide (2) 0.6 0.6 0.6 1.21.2 1.2 0.6 0.6 Antioxidant 0.1 Barium sulfate 16.6 16.4 16.4 13.4 14.920.4 16.6 Zinc oxide 5 5 5 4 4 24.9 5 5 Zinc stearate 5 5 5 5 5 5 Zincsalt of 1 1 1 0.1 0.1 1 1 1 pentachlorothiophenol VulcanizationTemperature (° C.) 155 155 155 155 155 155 155 155 conditions Time(min.) 10 10 10 13 13 10 10 10

Details on the ingredients shown in Tables 1 and 2 are given below.

-   Polybutadiene A: Available under the trade name “BR 01” from JSR    Corporation-   Polybutadiene B: Available under the trade name “BR 51” from JSR    Corporation-   Zinc acrylate: Available from Nippon Shokubai Co., Ltd.-   Organic peroxide (1): Dicumyl peroxide, available under the trade    name “Percumyl D” from NOF Corporation-   Organic peroxide (2): A mixture of 1,1-di(t-butylperoxy)-cyclohexane    and silica, available under the trade name “Perhexa C-40” from NOF    Corporation-   Water: Distilled water, from Wako Pure Chemical Industries, Ltd.-   Antioxidant: 2,2′-Methylenebis(4-methyl-6-butylphenol), available    under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical    Industry Co., Ltd.-   Barium sulfate: Available under the trade name “Barico #300” from    Hakusui Tech-   Zinc oxide: Available under the trade name “Zinc Oxide Grade 3” from    Sakai Chemical Co., Ltd.-   Zinc stearate: Available under the trade name “Zinc Stearate G” from    NOF Corporation-   Zinc salt of pentachlorothiophenol: Available from ZHEJIANG CHO & FU    CHEMICAL.

Formation of Intermediate Layer and Cover

An intermediate layer material formulated as shown in Table 3 wasinjected-molded over the two-layer core obtained above, thereby givingan intermediate layer-encased sphere. Next, using the cover materialformulated as shown in Table 3, a cover (outermost layer) wasinjection-molded over the intermediate layer-encased sphere, therebyproducing a golf ball having an intermediate layer and a cover(outermost layer) over the core.

TABLE 3 Resin materials (pbw) I II III IV V T-8295 75 100 T-8290 25 75T-8283 25 Himilan ® 1706 35 Himilan ® 1557 15 Himilan ® 1605 50 AN431920 AN4221C 80 Hytrel ® 4001 11 11 11 Titanium oxide 3.9 3.9 3.9Polyethylene wax 1.2 1.2 1.2 Isocyanate compound 7.5 7.5 7.5Trimethylolpropane 1.1 Magnesium stearate 60 Calcium hydroxide 1.5Magnesium oxide 1 Polytail H 8

Details on the materials shown in Table 3 are as follows.

-   T-8295, T-8290, T-8283:    -   MDI-PTMG type thermoplastic polyurethanes available from DIC        Bayer Polymer under the trademark Pandex.-   Himilan® 1706, Himilan® 1557, Himilan® 1605:    -   Ionomers available from DuPont-Mitsui Polychemicals Co., Ltd.-   AN4319: An unneutralized ethylene-methacrylic acid-ester component    terpolymer (from DuPont-Mitsui Polychemicals Co., Ltd.)-   AN4221C: An unneutralized ethylene-acrylic acid copolymer (from    DuPont-Mitsui Polychemicals Co., Ltd.)-   Hytrel 4001: A polyester elastomer available from DuPont-Toray Co.,    Ltd.-   Polyethylene wax: “Sanwax 161P” from Sanyo Chemical Industries, Ltd.-   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate-   Magnesium stearate: “Magnesium Stearate G” from NOF Corporation-   Calcium hydroxide: “Calcium Hydroxide CLS-B” from Shiraishi Calcium    Kaisha, Ltd.-   Magnesium oxide: “Kyowamag MF 150” from Kyowa Chemical Industry Co.,    Ltd.-   Polytail H: Available from Mitsubishi Chemical Corporation

At this time, dimples having the parameters shown in Table 4 below wereformed on the cover surface in the respective Working Examples andComparative Examples. Six types of dimples of differing diameters asshown in Table 4 were arranged on the golf balls in each of the WorkingExamples and Comparative Examples, and set to the same surface coverageratio SR.

TABLE 4 No. Number of dimples Diameter (mm) SR (%) 1 12 4.6 81 2 234 4.43 60 3.8 4 6 3.5 5 6 3.4 6 12 2.6 Total 330

Dimple Definitions

-   -   Diameter: Diameter of flat plane circumscribed by edge of dimple        (mm).    -   SR: Sum of individual dimple areas as a percentage of the total        surface area of a hypothetical sphere were the golf ball to have        no dimples thereon (unit: %)

Two dimple shapes were used. Dimple A (FIG. 1) was used in WorkingExamples 1, 2, 4 and 5 and Comparative Examples 1 to 3, and Dimple B(FIG. 2) was used only in Working Example 3. Of the six types of dimplesof differing diameter in Table 4, the structures of the typical dimpleshaving a diameter of 4.4 mm were as follows.

Dimple A

In the cross-sectional shape in FIG. 1, the depth L at the deepest pointis 0.150 mm.

Dimple B

In the cross-sectional shape in FIG. 2, the depth H at the center pointC is 0.097 mm, the depth D at the deepest point is 0.131 mm, thedistance from the outer peripheral edge E to the position of the deepestpoint, relative to an arbitrary distance of 100 from the outerperipheral edge E to the center point C, is 39, the radius of curvatureR is 0.5 mm, and the edge angle A2 is 10.5°.

For each of the resulting golf balls, properties such as the corehardness profile, thicknesses and material hardnesses of the respectivelayers, and the surface hardnesses of various layer-encased spheres wereevaluated by the methods described below. The results are shown in Table5.

Core Hardness Profile

The indenter of a durometer was set so as to be substantiallyperpendicular to the spherical surface of the core, and the core surfacehardness in terms of JIS-C hardness was measured as specified in JISK6301-1975.

To obtain the cross-sectional hardnesses at the center and otherspecific positions of the core, the core was hemispherically cut so asform a planar cross-section, and measurements were carried out bypressing the indenter of a durometer perpendicularly against thecross-section at the measurement positions. These hardnesses areindicated as JIS-C hardness values.

The Shore D hardnesses at the center of the inner core layer and thesurface of the outer core layer were measured with a type D durometer inaccordance with ASTM D2240-95.

Diameters of Inner Core Layer, Outer Core Layer-Encased Sphere andIntermediate Layer-Encased Sphere

The diameters at five random places on the surface were measured at atemperature of 23.9±1° C. and, using the average of these measurementsas the measured value for a single inner core layer, outer corelayer-encased sphere (entire core) or intermediate layer-encased sphere,the average diameter for five measurement specimens was determined.

Ball Diameter

The diameters at five random dimple-free areas on the surface of a ballwere measured at a temperature of 23.9±1° C. and, using the average ofthese measurements as the measured value for a single ball, the averagediameter for five measured balls was determined.

Deflections of Inner Core Layer, Outer Core Layer-Encased Sphere,Intermediate Layer-Encased Sphere and Ball

An inner core layer, outer core layer-encased sphere (overall core),intermediate layer-encased sphere or ball was placed on a hard plate andthe amount of deflection when compressed under a final load of 1,275 N(130 kgf) from an initial load of 98 N (10 kgf) was measured. The amountof deflection here refers in each case to the measured value obtainedafter holding the test specimen isothermally at 23.9° C.

Material Hardnesses of Intermediate Layer and Cover (Shore D Hardnesses)

The intermediate layer and cover-forming resin materials were moldedinto sheets having a thickness of 2 mm and left to stand for at leasttwo weeks, following which the Shore D hardnesses were measured inaccordance with ASTM D2240-95.

Surface Hardnesses of Intermediate Layer-Encased Sphere and Ball (ShoreD Hardnesses)

Measurements were taken by pressing the durometer indenterperpendicularly against the surface of the intermediate layer-encasedsphere or ball (i.e., the surface of the cover). The surface hardness ofthe ball (cover) is the measured value obtained at dimple-free places(lands) on the ball surface. The Shore D hardnesses were measured with atype D durometer in accordance with ASTM D2240-95.

Pressed Area

Measurement of the pressed area PS of a golf ball was carried out byplacing pressure-sensitive paper (Prescale pressure measurement film formedium pressure, available from Fujifilm Corporation) on a flat surface,and setting a golf ball from the respective Working Examples andComparative Examples thereon. Next, using a model 4202 tester fromInstron Corporation, loads of 6,864 N (700 kgf) and 1,961 N (200 kgf)were applied to these golf balls, and the total area of the portion ofthe pressure-sensitive paper that became colored due to contact with thegolf ball was measured. The area of the colored portion was determinedusing the FPD-9270 Prescale Pressure Image Analysis System (FujifilmCorporation). In each case, the pressed area is the result ofmeasurement at a single arbitrary position on the golf ball.

TABLE 5 Example Comparative Example 1 2 3 4 5 1 2 3 Inner core Materialrubber rubber rubber rubber rubber rubber rubber rubber layer Diameter(mm) 35.20 35.20 35.20 23.40 23.40 21.50 35.20 35.20 Weight (g) 26.526.5 26.5 7.8 7.8 6.0 26.5 26.5 Specific gravity 1.159 1.159 1.159 1.1631.163 1.159 1.159 1.159 Deflection (mm) 4.3 4.3 4.3 5.7 6.3 6.0 4.3 4.3Outer core Material rubber rubber rubber rubber rubber rubber rubberrubber layer Thickness (mm) 1.67 1.67 1.67 7.63 7.63 8.50 1.67 1.67Outer core Diameter (mm) 38.54 38.54 38.54 38.65 38.65 38.50 38.54 38.54layer- Weight (g) 34.8 34.8 34.8 35.1 35.1 34.8 34.8 34.8 encasedSpecific gravity 1.159 1.159 1.159 1.159 1.159 1.159 1.159 1.159 sphereDeflection (mm) 3.6 3.7 3.7 3.0 3.6 3.7 3.9 3.6 Core Center hardness: Cc(JIS-C) 53 53 53 57 54 55 53 53 hardness (Shore D) 32 32 32 35 33 34 3232 profile Hardness at position 5 mm 61 61 61 65 60 60 61 61 from,center: C5 (JIS-C) Hardness at position 10 mm 64 64 64 70 64 60 64 64from center: C10 (JIS-C) Hardness at position 15 mm 78 78 78 80 75 70 7878 from, center: C15 (JIS-C) C10 − Cc (JIS-C) 11 11 11 13 10 5 11 11 C5− Cc (JIS-C) 8 8 8 8 6 5 8 8 C10 − C5 (JIS-C) 3 3 3 5 4 0 3 3 C15 − C10(JIS-C) 14 14 14 10 11 10 14 14 Cs − C15 (JIS-C) 13 8 8 12 14 14 −3 13Surface hardness: Cs (JIS-C) 91 86 86 92 89 84 75 91 (Shore D) 61 57 5762 60 56 49 61 Outer core layer surface − 38 33 33 35 35 29 22 38 innercore layer center: Cs − Cc (JIS-C) (Shore D) 29 25 25 27 27 22 17 29 Cs− C10 (JIS-C) 27 22 22 22 25 24 11 27 (Cs − C10)/(C10 − Cc) 2.5 2.0 2.01.7 2.5 4.8 1.0 2.5 (Deflection of inner core layer)/(Deflection 1.2 1.21.2 1.9 1.8 1.6 1.1 1.2 of outer core layer-encased sphere) IntermediateMaterial I I I I I I I II layer Thickness (mm) 1.26 1.26 1.26 1.21 1.211.28 1.26 1.26 Specific gravity 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95Material hardness (Shore D) 62 62 62 62 62 62 62 55 IntermediateDiameter (mm) 41.06 41.06 41.06 41.06 41.06 41.06 41.06 41.06 layer-Weight (g) 40.7 40.7 40.7 40.7 40.7 40.8 40.7 40.7 encased Deflection(mm) 2.9 3.0 3.0 2.7 3.1 3.0 3.2 3.1 sphere Surface hardness (Shore D)68 68 68 68 68 68 68 61 Intermediate layer surface hardness − 7 11 11 1111 12 19 0 Outer core layer surface hardness (Shore D) (Deflection ofinner core layer)/(Deflection 1.5 1.4 1.4 1.4 1.4 2.0 1.3 1.4 ofintermediate layer-encased sphere) Cover Material III III III V V IIIIII IV Thickness (mm) 0.82 0.82 0.82 0.82 0.82 0.82 0.82 0.82 Specificgravity 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 Material hardness (ShoreD) 53 53 53 47 47 53 53 56 Dimples A A B A A A A A Ball Diameter (mm)42.70 42.70 42.70 42.70 42.70 42.70 42.70 42.70 Weight (g) 45.7 45.745.7 45.6 45.6 45.7 45.7 45.7 H: deflection (mm) 2.7 2.8 2.8 2.4 2.8 2.83.0 2.9 Surface hardness (Shore D) 65 65 65 59 59 65 65 67 (Deflectionof inner core layer)/ 1.6 1.5 1.5 2.4 2.3 2.1 1.4 1.5 (Deflection ofball) Ball surface hardness − −3 −3 −3 −9 −9 −3 −3 6 Intermediate layersurface hardness (Shore D) Intermediate layer thickness − 0.44 0.44 0.440.39 0.39 0.46 0.44 0.44 Cover thickness (mm) Deflection of Inner corelayer − 1.6 1.5 1.5 3.3 3.5 3.2 1.3 1.4 Deflection of ball (mm) S:Hypothetical planar area (mm²) 1,432 1,432 1,432 1,432 1,432 1,432 1,4321,432 PS7: Pressed area when loaded at 6,864 N (mm²) 236 243 276 217 243243 257 250 PS2: Pressed area when loaded at 1,961 N (mm²) 70 73 81 6473 73 77 75 Formula 1: PS7/S/H × 100 (mm⁻¹) 6.09 6.05 6.88 6.30 6.076.05 5.99 6.02 Formula 2: PS2/S/H × 100 (mm⁻¹) 1.82 1.81 2.02 1.87 1.821.81 1.80 1.80

In addition, the flight performance (W#1) and spin performance onapproach shots of the golf balls obtained in the respective Examples ofthe invention and the Comparative Examples were evaluated according tothe criteria indicated below. The results are shown in Table 6.

Flight Performance (W#1 Shots)

A driver (W#1) was mounted on a golf swing robot, and the distancetraveled by the ball when struck at a head speed (HS) of 45 m/s wasmeasured and rated according to the criteria shown below. The club was aTourStage X-Drive709 D430 driver (2013 model, loft angle, 9.50). Thishead speed corresponds to the average head speed of mid- to high-levelamateur golfers.

Rating Criteria:

-   -   Good: Total distance was 225.0 m or more    -   NG: Total distance was less than 225.0 m

Spin Performance on Approach Shots

A sand wedge was mounted on a golf swing robot, and the spin rate of theball when hit at a head speed (HS) of 35 m/s was rated according to thefollowing criteria.

Rating Criteria:

-   -   Good: Spin rate was 5,900 rpm or more    -   NG: Spin rate was less than 5,900 rpm

TABLE 6 Comparative Example Example 1 2 3 4 5 1 2 3 Flight Spin rate3,083 3,113 3,023 3,251 3,188 3,105 3,185 3,065 performance (rpm) W#1Total 226.4 225.9 226.3 225.4 225.2 224.7 224.0 226.6 HS, 45 m/sdistance (m) Rating good good good good good NG NG good Performance Spinrate 5,963 5,955 6,045 6,542 6,286 5,945 5,911 5,734 on approach (rpm)shots Rating good good good good good good good NG

The following observations are based on the test results in Table 6.

In Comparative Example 1, the inner core layer has a small diameter. Asa result, the initial velocity when struck with a driver (W#1) was slowand a good distance was not achieved.

In Comparative Example 2, the surface hardness of the outer core layerwas low. As a result, the spin rate-lowering effect on shots with a W#1was inadequate and a good distance was not achieved.

In Comparative Example 3, because the cover was harder than theintermediate layer, the ball was not sufficiently receptive to spin onapproach shots and so the desired spin effect on approach shots was notobtained.

Japanese Patent Application No. 2015-172780 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A multi-piece solid golf ball comprising a rubber two-layer coreconsisting of an inner layer and an outer layer, a cover, and at leastone intermediate layer between the core and the cover, wherein the innercore layer has a diameter of at least 22 mm, the value obtained bysubtracting a center hardness of the inner core layer from a surfacehardness of the outer core layer, expressed in terms of JIS-C hardness,is at least 25, the ball satisfies the relationship A/B≤2.0, where A isthe deflection (mm) of the inner core layer when compressed under afinal load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf)and B is the deflection (mm) of the two-layer core when compressed undera final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf),and the intermediate layer has a higher material hardness than thecover, wherein the multi-piece solid golf ball satisfies the conditions:C5−Cc≤Cs−C15C10−C5≤Cs−C15C5−Cc≤C15−C10, and1.5≤(Cs−C10)/(C10−Cc)≤4, where Cc is the JIS-C hardness at a center ofthe core, C5 is the JIS-C hardness at a position 5 mm from the corecenter, C10 is the JIS-C hardness at a position 10 mm from the corecenter, C15 is the JIS-C hardness at a position 15 mm from the corecenter, and Cs is the JIS-C hardness at a surface of the core.
 2. Thegolf ball of claim 1, wherein the core has a hardness profile whichsatisfies the conditions:1≤C10−Cc≤15,  (i)C10−Cc<Cs−C10,  (ii)18≤Cs−C10,  (iii)Cs≥80, and  (iv)Cc≥50.  (v)
 3. The golf ball of claim 1 which satisfies the condition:25≤Cs−Cc≤45.  (vii)
 4. The golf ball of claim 1 which satisfies thecondition:C10−C5≤C5−Cc≤Cs−C15≤C15−C10.  (viii)
 5. The golf ball of claim 1 whichsatisfies the relationship A/C≤1.9, where C is the deflection (mm) of asphere consisting of the core encased by the intermediate layer when thesphere is compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf).
 6. The golf ball of claim 1 whichsatisfies the relationships A/H≤2.0 and A−H≤2.5, where H is thedeflection (mm) of the ball when compressed under a final load of 1,275N (130 kgf) from an initial load of 98 N (10 kgf).
 7. The golf ball ofclaim 1 which satisfies the condition:PS ₇ /S/H×100≥5.90(mm⁻¹),  (ix) where PS₇ is the pressed area (mm²),defined as the area of the golf ball that comes into contact with a flatsurface, when the ball is subjected to a load of 6,864 N (700 kgf), S isthe hypothetical planar area (mm²), defined as the surface area of across-sectional circle along the ball diameter were the surface of theball to be entirely free of dimples, and H is the deflection (mm) of theball when compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf).
 8. The golf ball of claim 1 whichsatisfies the condition:PS ₂ /S/H×100≥1.70(mm⁻¹),  (x) where PS₂ is the pressed area (mm²),defined as the area of the golf ball that comes into contact with a flatsurface, when the ball is subjected to a load of 1,961 N (200 kgf), S isthe hypothetical planar area (mm²), defined as the surface area of across-sectional circle along the ball diameter were the surface of theball to be entirely free of dimples, and H is the deflection (mm) of theball when compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf).